1. Field of the Invention
The present invention relates to an antenna having a stripline splitter circuit.
2. Description of the Related Art
Stripline splitter circuits are employed in flat antennas to feed signal power to an array of antenna elements. Flat antennas of this type are useful in, for example, wireless communication systems that link computing devices or other electronic devices within a building.
A flat antenna described by Yamami in Japanese Patent Application Publication No. 7-297630 (paragraphs 0013-0017 and FIG. 1) has an array of antenna elements formed in a flat dielectric body. The splitter circuit is a network of feeder lines laid out between the antenna elements to carry signal power to and from the antenna elements. The layout is plane-symmetrical, symmetrically equivalent parts of the splitter circuit being aligned in the direction of the electric field generated by the antenna elements. The power feed point of the splitter circuit is offset from the plane of symmetry by one-fourth of the effective wavelength of the transmitted or received signal in the electric field direction. This allows the electric fields of the individual antenna elements to reinforce each other while causing unwanted electrical couplings between symmetrical pairs of antenna elements and power lines to cancel out, thereby reducing the occurrence of sidelobes in the field plane and improving the directional symmetry of the electric field.
Another flat antenna, described by Nishi et al. in ‘Development of Millimeter-Wave Video Transmission System, Development of Antenna’ (proc. 2001 Asia-Pacific Microwave Conf., Vol. 2, pp. 509-512, December 2001), has an 8×8 array of circular waveguides 3.2 mm in diameter that radiate or receive signals in the 66-GHz band. The splitter circuit is formed in a dielectric substrate sandwiched between the upper and lower halves of the body of the antenna.
FIG. 1 illustrates, in plan view, the splitter circuit in a 2×2 antenna unit in this flat antenna, indicating the positions of four radiating elements 1-4, four striplines 11-14, and four feeder electrodes 21-24 in a Cartesian coordinate system with X and Y axes. One end of the first stripline 11 is joined to the second stripline 12 at a branching point P1 in the middle of the second stripline 12. The two ends of the second stripline 12 are joined to the third and fourth striplines 13, 14 at branching points P2, P3 in the middles of those striplines 13, 14. The ends of the third stripline 13 are joined to the ends of feeder electrodes 21, 22 which extend in the negative Y direction into the first and second radiating elements 1, 2. The ends of the fourth stripline 14 are joined to the ends of feeder electrodes 23, 24 which extend in the negative Y direction into the third and fourth radiating elements 3, 4.
FIG. 2 shows two adjacent antenna units U1, U2 with an alternative layout in which the feeder electrodes 21, 22, 23, 24 extend in the negative X direction. The first striplines 11 of both antenna units receive power from an input stripline 10 at an input branching point P0.
A requirement of the splitter circuits in FIGS. 1 and 2 is that the four paths from the input stripline to the radiating elements 1-4 via the first, second, and third branching points P1, P2, P3 must have equal power splitting ratios and uniform phase delays. Basically, this means that the four branched stripline paths must have equal total electrical lengths. Consequently, the first branching point P1 must be disposed between the first and second radiating elements 1 and 2, which constrains the spacing of the array.
To reduce variations in stripline impedance, and to suppress unwanted radiation caused by stray coupling from the striplines into the waveguide windows that form the radiating apertures of the circular waveguide array antenna, the layout of the striplines on the surface must be properly balanced with respect to the ground plane, which is situated in a separate layer below the striplines. Specifically, the striplines must not approach the edges of the ground plane, which is bored with holes having diameters equal to the diameters of the waveguide windows, too closely.
This condition is met in FIG. 1 as follows: the first and second striplines 11, 12 are straight in the vicinity of the first branching point P1, where they form a T-shaped triple junction; the third and fourth striplines 13, 14 are straight over their entire lengths; the first stripline 11 keeps a distance d1 from the edge of radiating element 2; the second stripline 12 keeps a similar distance d2 from the edge of radiating element 1; the first branching point P1 is offset in the negative X direction from an imaginary line S joining the second and third branching points P2, P3; the imaginary line S coincides with the midline of the array in the Y direction. With these arrangements, the spacing of the radiating elements 1-4 in the array is reducible to 4.1 mm.
In FIG. 2 the above condition is met as follows: the first stripline 11 follows the edge of the first or second radiating element 1 or 2 partway therearound, maintaining a distance d1 from the aperture of the first or second radiating element 1 or 2; the second, third, and fourth striplines 12, 13, 14 are straight in the vicinities of the first, second, and third branching points; the third stripline 13 has terminal parts that follow edges of the first and second radiating elements 1, 2 partway therearound, maintaining predetermined distances d3, d4 from the apertures of the first and second radiating elements 1, 2; the fourth stripline 14 has terminal parts that follow the edges of the third and fourth radiating elements 3, 4 partway therearound, maintaining predetermined distances d3, d4 from the apertures of the third and fourth radiating elements 3, 4; distances d1, d3, and d4 are mutually equal; the first branching point P1 is offset in the negative X direction from an imaginary line S joining the second and third branching points P2, P3. With these arrangements, the array spacing is reducible to 4.4 mm.
The need to maintain the predetermined distances d1, d2, d3, d4 and to provide space for the first branching point between the first and second radiating elements 1, 2 precludes further reductions in the spacing of the radiating elements in these layouts. This has been an obstacle to the improvement of antenna performance.
Accordingly, there has been an unfulfilled need to provide an array antenna with a stripline splitter circuit capable of aligning the phases of power fed to the radiating elements, shortening the electrical path lengths, and narrowing the spacing between radiating elements.